JPH0320053B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Field This invention relates to a permanent magnet used in a medical nuclear magnetic resonance tomography device (hereinafter referred to as NMR-CT) that can obtain a cross-sectional image of an object and depict the properties of the tissue. The present invention relates to a magnetic field generating device that generates a strong, highly accurate, and uniform static magnetic field within a large air gap.

BACKGROUND ART
In order to obtain the desired tomographic image by inserting the device into an air gap that generates a strong magnetic field of 10 KG, this magnetic field must be strong, uniform, and stable with an accuracy of 10 ^-4 or less. Known magnetic field generators include normal conducting magnets made of conductive wires made of copper or aluminum wrapped around them in a cylindrical shape, and superconducting magnets that use special conducting wires cooled to a temperature close to absolute zero. .

The former is structurally inexpensive, but it requires a huge amount of electricity and cooling water to generate a sufficiently strong magnetic field, and has problems such as high running costs.
On the other hand, the latter type of superconducting magnet has the advantage of consuming less power and being able to generate a strong magnetic field in a small size, but it is essential to use expensive liquid helium as a coolant, which increases the running cost as well as the so-called initial cost. There are significant problems.

The applicant previously proposed a magnetic field generator using a permanent magnet, which has a magnetic field strength equal to or higher than that of the above-mentioned normal conducting magnet, has low running costs, and has less magnetic field leakage than superconducting magnets and normal conducting magnets. (Akira Jigan
No. 59-53575).

As shown in FIG. 6, the magnetic field generating device described above includes a pair of permanent magnet components 1, 1, with magnetic pole pieces 2, 2 fixed to one end of each of them facing each other, the other end of which is connected with a yoke 3, It is configured to generate a static magnetic field in the air gap 4 between the magnetic pole pieces 2, 2, and the pair of magnetic pole pieces 2, 2 include:
An annular protrusion 5 having a substantially trapezoidal cross section with a predetermined inner diameter and height is protruded from the periphery of the opposing surface, and an annular protrusion 5 having a substantially trapezoidal cross section with a predetermined inner diameter and height is provided at the center of the pole pieces 2, 2. A convex protrusion 6 is provided to generate a strong, uniform and highly accurate magnetic field within the air gap 4.

However, in the above-mentioned magnetic field generator, the magnetic flux from the magnetic poles tends to leak out of the air gap, and on the vertical line of the air gap center, the closer to the magnetic pole surface the higher the magnetic field strength becomes. In order to obtain a magnetic field, it is necessary to increase the distance between the magnetic poles and the area of the magnetic poles, thereby creating an air gap several times larger than the magnetic field space to be used, which poses a problem in that the magnetic field circuit cannot be miniaturized.

Purpose of the Invention In view of the current situation, the present invention aims to provide a magnetic field generating device that generates a highly accurate, uniform, and stable magnetic field in a required air gap, and that has a magnetic field circuit that further expands the uniform magnetic field. The purpose of the present invention is to provide a magnetic field generating device that can effectively concentrate the magnetic flux from the permanent structure, reduce the weight of the permanent magnet, and achieve a reduction in size and weight.

Structure and Effects of the Invention As a result of various studies aimed at creating a magnetic circuit that can obtain a highly accurate, uniform, and stable magnetic field and has little leakage magnetic flux, the present invention has been made based on the results that the magnetic pole surfaces of a pair of permanent magnet components are aligned with the air gap. By arranging a separate permanent magnet structure around the flat permanent magnet structure so as to form a concave curved surface, the magnetic field uniformity and magnetic field strength within the magnetic circuit gap are significantly improved.
This is based on the discovery that it is possible to expand the uniform magnetic field area with high precision.

That is, this invention magnetically couples a pair of permanent magnet structures facing each other by forming a gap with a yoke, fixes a magnetic pole piece to the surface of each permanent magnet structure facing the gap, and generates a magnetic field in the gap. In a magnetic field generating device, a flat central permanent magnet and an outer peripheral permanent magnet installed or arranged around the central permanent magnet in an inclined manner, each pair is magnetized in a direction perpendicular to the magnetic pole surface. This magnetic field generating device is characterized in that the permanent magnet magnetic pole surface forms a concave curved surface with respect to the air gap.

Preferred Embodiment of the Invention The magnetic circuit can have any configuration as long as a pair of opposing permanent magnet structures are magnetically coupled with a yoke by forming an air gap. It is desirable to appropriately select the arrangement of the permanent magnet structure, etc., depending on the shape and dimensions of the magnet, the size of the required air gap, etc.

Various configurations and shapes can be used for the yoke constituting the magnetic circuit. For example, the yoke 10 shown in FIG. When a permanent magnet structure according to the present invention (not shown) is attached to the opposing inner surfaces of the plate yokes 11, 11, the required air gap 13 is formed. The gap 13 thus formed has the advantage that the object to be diagnosed can be inserted more easily than between the columnar yokes 12 in either direction, and assembly and maintenance management are also facilitated.

In addition, although Fig. 2 shows an example in which square plates are used as the plate-shaped yokes 11, 11, any shape such as a disk shape can be applied. It is also good to use a rectangular plate.

Furthermore, it is desirable to select the shape, number, etc. of the columnar yoke 12 within a range that does not saturate the magnetic flux passing through it.

The yoke 14 shown in FIG.
6 are arranged facing each other, and both are connected by a cylindrical yoke 17, and the opposing disc-shaped yokes 15, 16
A permanent magnet structure (not shown) is attached to the inner surface to form a gap, and the gap is surrounded by the cylindrical yoke 17, in order to insert the object to be diagnosed into the cylindrical yoke 17. There is an advantage that there is almost no leakage magnetic field from the yoke 14 simply by providing the opening 17a.

The yoke 18 shown in FIG. 4 is a square cylindrical yoke, with the opening 19 directed horizontally, and the upper and lower yoke surfaces 18a, 1
A pair of permanent magnet structures (not shown) are attached to the inner surface of the magnet 8b to form a gap.

In this invention, a pair of permanent magnet structures are attached to opposing yoke surfaces of yokes of various configurations, such as the above-mentioned yoke. If a concave magnetic pole surface can be formed by the outer periphery permanent magnet installed or arranged around the outer periphery of the permanent magnet at an angle, the shape of the central permanent magnet can be of various shapes such as a disk shape or a polygonal shape. Depending on the shape of the central permanent magnet, the required uniformity of the magnetic field in the gap, etc., the permanent magnets may be arranged around the circumference or in a required number at radial positions from the center of the central permanent magnet.

In addition, since all permanent magnets are usually assembled into a desired shape using a magnetization assembly method using a plurality of block-shaped magnets, it is desirable to have a shape that takes assembly workability into consideration.
For example, if the central permanent magnet is made into a polygonal shape and arranged in an outer peripheral permanent magnet having a shape that can come into contact with each side of the outer periphery, the permanent magnet assembly can be easily assembled.

In this invention, as described above, on the magnetic pole surface of the permanent magnet structure in which the opposing magnetic pole surface is a concave curved surface,
A magnetic pole piece having a concave curved surface is installed, and the surface of the magnetic pole piece facing the air gap may form a concave curved surface considering the required uniform magnetic field, and the back surface that contacts the permanent magnet is attached to the central permanent magnet. It is necessary to select an appropriate value depending on the shape, number, etc. of the outer peripheral permanent magnet.

For example, the magnetic pole piece 20 shown in FIG. 5 has a so-called deep-dish shape with a required concave curved surface, and the contact surface with the permanent magnet is formed between the central permanent magnet whose permanent magnet structure is octagonal and its contact surface with the permanent magnet. Considering that it is composed of trapezoidal outer peripheral permanent magnets that contact each side of the outer periphery and can be adjacent to each other in the circumferential direction, as shown in the bottom drawing of Figure B, 8.
It is composed of a rectangular flat part 21 and eight trapezoidal inclined parts 22 provided around the outer periphery of the outer peripheral part 21 of the flat part 21, ensuring reliable magnetic path formation with the permanent magnet structure.

Further, as shown in Fig. C, a protrusion 24 is provided at the center of the plane portion 23 of the magnetic pole piece 20 on the side opposite to the air gap.
It is also possible to adopt a configuration that improves the uniformity of the magnetic poles within the air gap.

The permanent magnet used in the magnetic pole generator of this invention is
Ferrite magnets, alnico-based magnets, and rare earth cobalt-based magnets can be used, but the applicant previously proposed (Japanese Patent Application No. 145072/1982) that R should be a light rare earth material that is rich in resources, mainly Nd and Pr. Use, B,
By using a Fe-BR-based permanent magnet that has Fe as its main component and exhibits an extremely high energy product of 25 MGO ₈ or more, it can be significantly miniaturized.

DISCLOSURE OF THE INVENTION BASED ON THE DRAWINGS FIG. 1 is a cross-sectional top view and a BB sectional view of a magnetic circuit used in a magnetic field generating device according to the present invention.

In the magnetic circuit, the yoke 30 has a configuration in which a pair of square plate yokes 31 and 32 are arranged facing each other and connected by four cylindrical yokes 33, and the plate yokes 31, Permanent magnet structures 40, 50 according to the present invention are attached to the opposing inner surfaces of the magnets 32 to form the required air gap 60.

In the yoke 30, cylindrical yokes 33 erected at the four corners of the lower plate-shaped yoke 32 penetrate through holes at the four corners of the upper plate-shaped yoke 31, and the cylindrical yoke 33 protrudes. The cover 34, which is a headed cylinder, covers the upper plate-shaped yoke 3.
The upper plate-shaped yoke 31 is fixed to the outer periphery of the hole 1 with a bolt 35, and the adjustment bolt 36 screwed into the center part.
The upper plate-shaped yoke 31 is placed on the upper end of the cylindrical yoke 33 and can be slightly moved vertically by screwing the adjustment bolt 36 back and forth. It is configured such that the distance between the cover 34 and the columnar yoke 33 can be adjusted by fixing bolts 37 provided around the adjustment bolts 36.
It is a configuration that can be fixed.

On the opposing surfaces of the upper and lower plate yokes 31 and 32,
Permanent magnet structures 40, 50 are attached via disk yokes 38, 39 having holes in the center, and include central permanent magnets 41, 51 and outer peripheral permanent magnets 43, 53.
The central permanent magnets 41, 51 are formed into an octagonal plate shape with magnetic force adjustment holes 42, 52 provided in the center, and eight approximately trapezoidal plate-shaped outer peripheral permanent magnets 43, 53 are formed. , are in contact with each side of the central permanent magnets 41, 51, and are provided around the periphery so as to form concave opposing surfaces by being inclined through trapezoidal wedge-shaped yokes 44, 54.

The permanent magnet structures 40 and 50 are constructed into the above shape by magnetizing and assembling a large number of block-shaped Fe-BR permanent magnets, and the magnetization direction is perpendicular to the magnetic pole surface. have.

The permanent magnet structures 40 and 50 facing each other with concave magnetic pole surfaces each have the same structure as in FIG. A sloped portion is provided around the circumference, and further,
Magnetic pole pieces 45 and 55 having circular protrusions 46 and 56 on opposing plane parts and having curved surfaces with a substantially concave cross section are attached, and a magnetic flux is directed into the air gap 60 formed between the pair of magnetic pole pieces 45 and 55. It has the function of efficiently focusing.

So-called tapered holes 70, 71 are provided in the center of the non-attached side surfaces of the plate yokes 31, 32, which communicate with the holes of the disk yokes 38, 39, and are connected to the center permanent magnet 41. , 51 magnetic force adjustment holes 42, 52
Magnetic force adjustment material 7 made of a cylindrical magnetic material to be inserted into
2 are installed around the holes of the disc yokes 38 and 39 so that the amount of insertion can be adjusted.

Tapered hole portions 70, 7 of plate yokes 31, 32
1 is arranged to reduce the weight of the yoke and to prevent the magnetic force adjustment material 72 from being exposed to the outside, focusing on the fact that the amount of magnetic flux in the center of the plate yokes 31 and 32 is reduced. .

Further, the uniformity, distribution, etc. of the magnetic field can be adjusted using the adjusting bolt 36 that comes into contact with the upper end of the columnar yoke 33 that adjusts the facing distance between the permanent magnet structures 40 and 50.
In addition to the magnetic field adjustment material 72 that fits into the center of the permanent magnet structures 40 and 50 and adjusts the amount of magnetic flux short circuit from the central permanent magnets 41 and 51, the disc yoke 3
8, 39 in the horizontal direction, or by slightly moving the permanent magnet structures 40, 50 depending on their thickness, etc.
It is also possible to carry out leveling, and each means may be selected appropriately.

Furthermore, disc yokes 38, 39 and wedge-shaped yokes 44, 5
4 may be integrally formed.

In the magnetic circuit having the above configuration, the magnetic flux generated from the permanent magnet components 40 and 50 is transmitted to the magnetic pole piece 4.
5 and 55 to form a magnetic field converged in the same direction as the magnetization direction of the magnet, leakage magnetic flux is reduced,
A static magnetic field with extremely high uniformity is obtained within the air gap 60.

In particular, since the permanent magnet components 40, 50 each having a concave magnetic pole surface are provided with deep dish-shaped magnetic pole pieces 45, 55, the outer circumferential area of the magnetic pole pieces 45, 55 is as shown in FIG. A magnetic pole piece 2 with an annular protrusion 5 shown
This has the effect of significantly reducing leakage magnetic flux.

By the way, NMR-CT with the configuration shown in Figure 1
has a maximum energy product of 35MGO ₈ specific
When assembling Fe-BR-based permanent magnets and setting the facing distance of the magnetic pole pieces to 570 mm, the magnetic field strength at the center of the air gap was 1.5 kG.
A uniform magnetic field of less than 40 ppm could be obtained within 300 mm DSV, and the weight of the permanent magnet structure was 1.1 tons.

In addition, using permanent magnets with the same composition and magnetic properties,
In the conventional magnetic circuit having the configuration shown in FIG. 6, a permanent magnet of 1.5 tons was required to obtain a uniform magnetic field range and magnetic field strength comparable to that of the embodiment of the present invention described above.

Furthermore, we measured the amount of magnetic flux φ ₁ that passes through the yoke contact surface of the permanent magnet and the amount of magnetic flux 0 φ ₂ that is concentrated in the center of the air gap, that is, the space between the center planes of the magnetic pole pieces, and calculated the magnetic flux utilization rate (φ ₁ /φ ₂ ×100%), it was found that the magnetic flux utilization rate of the magnetic circuit of the present invention (FIG. 1) was 45%, and the magnetic flux utilization rate of the conventional magnetic circuit (FIG. 6) was 32%. It can be seen that the magnetic circuit according to the present invention has low leakage magnetic flux and extremely high magnetic efficiency.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional top view and a BB sectional view of a magnetic circuit used in a magnetic field generating device according to the present invention. 2 to 4 are perspective explanatory views showing the yoke of the magnetic circuit. FIG. 5 is a cross-sectional view and a bottom view showing a magnetic pole piece of a magnetic circuit according to the present invention, as well as a cross-sectional view of another magnetic pole piece. FIG. 6 is a longitudinal sectional view showing a conventional magnetic circuit. 10, 14, 18, 30... Yoke, 31, 32
... Plate-shaped yoke, 33 ... Cylindrical yoke, 34 ... Cover, 35 ... Bolt, 36 ... Adjustment bolt,
37... Fixing bolt, 38, 39... Disc yoke,
40,50...Permanent magnet structure, 42,52...
Tapered hole, 43, 53... Permanent magnet at outer periphery, 44, 54... Wedge-shaped yoke, 45, 55... Magnetic pole piece, 46, 56... Protrusion, 60... Gap, 7
0, 71...Tapered hole portion, 72...Magnetic field adjustment material.

Claims (1)

[Claims] 1. In a magnetic field generating device that magnetically couples a pair of permanent magnet structures facing each other with an air gap formed therein using a yoke, fixes a magnetic pole piece to the air gap facing surface of each permanent magnet structure, and generates a magnetic field in the air gap. , the magnetic pole surface of each pair of permanent magnets is magnetized perpendicularly to the magnetic pole surface by the flat central permanent magnet and the peripheral permanent magnet installed or arranged around the central permanent magnet in an inclined manner. , A magnetic field generating device characterized in that a concave curved surface is formed with respect to the air gap.